1. Field of the Invention
This invention relates to an incremental length measuring system comprising a scale member, which is provided with a measuring scale, a scanning unit having scanning element for scanning the scale and for generating measured-value signals in response to the scanning of the scale, and an analyzing unit for analyzing said measured-value signals, wherein said scale member comprises a reference track, which is parallel to the measuring scale and provided with a plurality of selectable reference marks associated with respective reference points, the scanning unit comprises a plurality of scanning elements associated with said reference marks, a selector switch is provided, which is movable in unison with the scanning unit and is operable by permanent magnet means, which are associated with any of the reference marks when it has been selected and are contained in a protective housing that contains the scale member and the scanning unit, and said selector switch is operable to permit a generation or delivery of a reference pulse in response to the scanning of a reference mark by said scanning unit only when said reference mark has been selected.
2. Description of the Prior Art
In such length measuring apparatuses it is possible to select a reference mark because this will permit a simpler manufacture of the measuring means and will permit a selection of a predetermined reference point without a need for an invasion into the housing in which the measuring means are encapsulated. Such reference point may correspond to the machine datum of a machine tool and in that case the measurements may be related to the machine datum. A selection of reference marks will make sense also in case of measurements which are to be related to such reference mark. Published German Application 18 14 785 discloses mechanical on-off switches, which in conjunction with the shifting of the scanning unit are actuated by means of stops and which activate the analyzing circuit only at preselected reference marks. German Patent Publication 25 40 412 discloses length measuring systems which are of the kind described first hereinbefore and comprise permanent magnets, which are adapted to be attached to the scale member at preselectable points and serve to actuate reed relays, which are integrated in the scanning unit. But the mounting of such permanent magnets on the scale member into the housing of an encapsulated length measuring apparatus requires an undesired invasion into the interior of the protective housing. In most cases the permanent magnet is clamped to the scale member so that the permanent magnet may be shifted or fall off during a prolonged operation. For this reason it has already been proposed to form the protective housing with a longitudinal bore, which is separate from the interior space in which the scale member and the scanning unit are accommodated, and to slidably insert the permanent magnet into that longitudinal bore from the end of the housing. The permanent magnet is held in position by spacers, which fill the bore as far as to both ends. In that case the assembling is expensive and the housing containing the scale member must be accessible at the ends to permit the permanent magnet and the spacers to be inserted into and pulled out of the bore.
For a higher accuracy of measurement it is desired that in case of a differential thermal expansion of the scale member, the protective housing and the structure on which the length measuring apparatus is mounted--that structure normally consists of the bed of a machine tool--the selected reference mark will not appreciably be shifted relative to the datum of the machine even in case of such differential thermal expansion. It has been attempted to fix the scale member in the housing only at the length center of the scale member and by means of an adhesive which has only a low elasticity and to yieldably mount the remaining portions of the scale member so that said portions can longitudinally shift relative to the protective housing. Besides, it is known to fix the protective housing to the carrying structure at the length center of the housing. In the previous practice the carrying structure was provided with rigid means for fixing the ends of the protective housing but linear elements which are capable of a linear adjustment, such as specially formed springs, hinges having crossing hinge axes, were provided between such fixing elements and the ends of the protective housing. Such a mounting is expensive and will not actually fulfill the desired purpose unless the rigid fixation of the protective housing and the fixation of the scale member to the protective housing are incidentally effected at the machine datum.